What is the best interpretation of the following statement?

“Neither slavery, nor involuntary servitude, except as a punishment for crime, whereof the party shall have been duly convicted, shall exist within the United States, nor any place subject to their jurisdiction.”

a. Slavery and involuntary servitude is outlawed except as a punishment for a crime in which the person has been convicted.
b. Slavery and involuntary servitude is subject to the jurisdiction area.
c. Slavery is a punishment for a crime.
d. Slavery and involuntary servitude is voluntary in a jurisdiction area.